The online retail giant has brought tens of thousands of workers to its campus in the South Lake Union neighborhood, overtaken the University of Washington as Seattle's biggest employer and lined up enough office space to roughly triple its headcount here. "A lot of people who have lived in Seattle for 10 or 20 years are getting pushed out, "says Jeff Reifman, a former Microsoft programmer who has criticized the ways Amazon is changing Seattle, including in a well-read essay last year on how the influx of male tech workers has skewed the dating scene. Mayor Ed Murray, who has a special committee seeking ways to provide affordable housing and avoid displacing longtime residents, last week announced another step: Amazon says it has more than 20,000 workers in Seattle, and estimates suggest it has enough office space built or planned to grow to more than 70,000, taking up a huge chunk of the city's commercial real estate. FareStart, a restaurant and catering business that trains homeless people for food-service careers, is across the street from a building under construction for Amazon. Amazon says 55 percent of its workers bus, bike or walk to work, and it notes it has given the city tens of millions of dollars for affordable housing, paid for a new street car and has contributed to nearly 100 charitable organizations. [...] the company has also brought a lot of people into an area that has relatively little housing or public transportation, though the city has added bus service and street cars, and light rail lines are being expanded.